sh: Kill off legacy UBC wakeup cruft.

This code was added for some ancient SH-4 solution engines with peculiar
boot ROMs that did silly things to the UBC MSTP bits. None of these have
been in the wild for years, and these days the clock framework wraps up
the MSTP bits, meaning that the UBC code is one of the few interfaces
that is stomping MSTP bits underneath the clock framework. At this point
the risks far outweigh any benefit this code provided, so just kill it
off.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>